Coating of metal surfaces



Patented July 4, 1950' UNITED STATES COATING 0F METAL SURFACES Paul Amundsen, Detroit, Mich, assignor, by

mesne assignments, to Parker-Rust Proof Company, Detroit, Miclm, a. corporation of Michigan No Drawing. Application September 4, 1948, Serial No. 47,937

11 Claims. 1

The present invention relates to the coating of metal surfaces and more particularly to an improved process for forming a substantially insoluble phosphate coating to retard oxidation or corrosion of the treated metal surface and to provide a strong rust and corrosion resistant phosphate coating on the surfaces of metals, such as ferrous metals, which have a tendency to oxidize and rust or corrode.

Phosphate containing solutions have been used previously to form a substantially insoluble rust and corrosion resistant phosphate coating on such surfaces. The formation of a phosphate coating of suitable thickness to provide the desired protection requires a substantial time period for contact between the surface to be protected and the single bath phosphate containing solution which often is too long to be commercially feasible. In such instances, the reaction is accelerated by the use of oxidizing agents in the phosphate solution bath. Both nitrates and. chlorates have been used as such oxidizing agents and while they are efieetive in reducing the time required to secure a phosphate coating of the required thickness, they -do not produce phosphate coatings having as good rust and corrosion resistant properties as do the phosphate solutions in which such oxidizin'g' agents are not used.

It is therefore, an object of the present invention to provide 'a novel process forthe coating of metal surfaces with a substantially insoluble phosphate coating having a high degree of resistance to rust and corrosion, the coating being formed in a relatively shorttime period, in two steps, and having increased rust and corrosion resistant properties, which cannot be obtained with the usual single bath processes.

It is a further object of the present invention to provide a novel process for the coating of metal surfaces with a substantially insoluble phosphate coating which provides for the eilicient utilization of the coating ingredients in the phosphate solution baths to provide a coating havin high rust and corrosion resistant properties at a rela tively higher rate and at a relatively lower cost than is possible by conventional single bath processes when operated to produce-a phosphate coat- 'ing having comparable properties.

Other objects of the invention will appear in the following description and appended claims. Before explaining the present invention in detail, it is to be understood that the invention is capable of other embodiments and of being practiced or carried out in various ways. It is also to be understood that the phraseolo'gy or terminology employed herein is for the purpose of description and not of limitation.

Conventional phosphate coatings-are formed by aprocess using asinglebath which contains phos phates, an oxidizing agent and other ingredients;

The metallic article to be coated is placed in such bath and the required reactive conditions are established and maintained until the desired coating is formed. The reactions which occur between the surfaces of the metallic article and the components of the bath produce a sludge which precipitates from the bath and reduces its effectiveness. In commercial operation, it is necessary to replace such baths frequently. Between the time when such baths are first put into operation and their replacement, the baths lose efliciency at a varying rate, depending upon the amount of metal dissolved from thesurfa'ces of the arti cles to be coated and the amount of precipitation of such metal from the bath as a salt formed by reaction with active salt forming constitu' ents of the bath. This makes accurate control very difllcult and requires the establishment of general control factors which do not permit the optimum utilization of the ingredients of. the bath.

As distinguishedfrom such conventional processes, the present invention utilizes a multiple bath process in which the solutions utilized in the separate baths are formulated and used under optimum conditions of reaction to provide a suitable adherent phosphate coating on the metallic article at a fast rate. The process of the present invention utilizes the constituent elements of the phosphate solutions efficiently and with a minimum of loss thereof due to sludging.

In accordance with the present invention, the properly cleaned metallic articles to be coated, such for example, as nuts, bolts, or the like, are immersed in a first bath which contains an acid phosphate coating solution havingv a high ferrous iron and low zinc content. The ratio of zinc to iron in this bath is maintained within the range of from approximately 1:10 to 1:2. In other words, the iron ranges from as much as ten times to as little as two times the amount of zinc used. I

After immersion in this bath for from 10 to 2 0 minutes, the preliminary reaction between the metallic articles and the bath has taken place and the articles are then removed from the bath and without drying or rinsing, are immersed in a second bath, for an equal lengthof time. The second bath is an acid phosphate coating solution having a high zinc and a low ferrous iron content. In this secondbath, the ratio of zinc to iron is maintained within the range of from approximately 2:1 to 10:1. In other words, the zinc ranges from as little as two times the amount of iron to as much as ten times the amount of H011.

As a result of such multiple bath process,- the articles are finished in the bathhav-ing the higher Zinc content. This gives a superior effect to that noted when the articles are processed in a single bath solution which after use has a relatively high ferrous iron content and a relatively low zinc content.

The articles most usually processed in such a bath are ferrous metal article-s. In the present process, most of the iron which is dissolved is found in the first bath, and the second bath is used only to provide the finishing rust and corrosion resistant coating. It has been found that the second and successive baths therefore accumulate but little iron.

Before the phosphate solution in the first bath attains an iron concentration, such that an iron salt precipitation will occur, a portion of the solution of the first bath is drawn off and discarded and a portion of the solution from the second bath is then added to the first bath to maize up for the solution discarded. The second bath is then replenished with fresh concentrated phosphate solution. The fresh replenishing concentrated solution is added to the second bath, thus maintaining it respectively high in zinc and rela tively low in iron content at all times. After the baths have been replenished in this manner, they are ready for another series of loads and the process may be repeated indefinitely.

In a typical instance, two loads were added every 20 minutes for 8 hours. At the end of 8 hours, 25% of the solution of the first bath was drawn off and discarded and 25% of the solution of the second bath was added to the first bath. The second bath was then brought to volume by the addition of fresh solution.

The following is an example of a commercial pilot run of the coating process embodying the present invention.

The solutions used in both baths contain the same ingredients, but as the process is operated the proportions of these ingredients are changed by the reaction with the metal of the materials to be treated. The ingredients are zinc dihydrogen phosphate, phosphoric acid, zinc chloride and ferrous chloride. The chlorides tend to suppress excessive hydrolysis and are used to keep the sludge formation at a minimum.

The acidity of the solutions were measured using N/ sodium hydroxide and titrating a 10 ml. sample using methyl orange and phenolphthalein indicators respectively. In the first bath, a typical solution has a free acid of 4.5, a total acid of 30.0 and a ratio of free acid to total acid of 1:7.0. In such first bath the ferrous iron content was 4 grams per liter, the zinc content was 0.8 gram per liter and the chloride (Cl-) content was 2.5 grams per liter.

The second bath had a slightly higher free acid content due to its proportionately higher zinc content. When similarly tested a typical solution used in the second bath had a free acid of 5.1, a total acid of 29.5, and a ratio of free acid to total acid of 1:5.8. The ferrous iron content of the second bath was 1 gram per liter, and the zinc content was approximately 8 grams per liter and the chloride (01-) was 2.9 grams per liter.

The concentrates from which both baths are made has the following typical composition in the following stated approximate per cents by weight:

Per cent ZnO 11.0' 75%H3PO4 4.6.5 ZnCla 6.0 FGClz 4.4 Water 32.1

Total 100.0

When used in the coating baths, the concentrate is diluted by the addition of water until a typical working solution will be found to contain the materials in the following approximate per cents by weights:

Per cent ZnO .33

H3PO4 1.40

ZnCh 0.20 Feclg .13 Water 97.94

Total 100 To make up a solution of working strength, approximately 3 gallons of the concentrate are added to 100 gallons of water and heated to approximately 200" F.

When first put in operation the solutions of all baths have an identical composition. After'the first few loads of metal objects are placed in the first bath, the iron content of this bath is built up rapidly so that in this first bath where the initial reactions occur, a high ferrous iron and a low zinc content will be found while the second and successive baths maintain the initial high zinc and low ferrous iron content.

The zinc and iron content in the concentrated solution may be varied to some extent. However, it is desirable that the iron content of the first bath should not be allowed to exceed approximately 5 grams per liter as the bath will become sludgy at higher concentrations. If the total acid in this bath is maintained constant, the zinc may be allowed to fall below 1 gram per liter. In the second bath, the maximum permissible iron should not exceed 1.5 grams per liter while the zinc content may be as high as 4 grams or even higher per liter.

In place of two separate baths, as here dise closed, it is contemplated that additional baths may be used in accordance with the present invention as the particularly novel feature of this invention lies in the use of a plurality of separate baths and coating solutions which may be separately controlled to effect the optimum reactions to produce satisfactory rust and corrosion resistant coatings.

In carrying out the process of this invention, it will be understood therefore that I may use more than two baths. Where, for example, I use 3 baths, the first two baths are used to give a partial coating and the last bath gives the finished coating. Solution from the first bath is discarded, solution from the second bath replaces the discarded solution of the first bath, while solution from the third bath replenishes the solution in the second bath and a new fresh solution is added to the third bath. This same sequence is employed where more than three baths are used.

While the foregoing contemplates the use of zinc phosphate in the coating solution, it has, been found that manganese phosphate may be used in place thereof. While oxidizing agents may be used in both baths to speed up the coating action, the use of such agents does not in general produce as high a corrosion resistance as is produced by the baths without these agents.

The phosphate coating formed by the multiple bath process of the present invention has been tested in a standard salt spray test run at degrees F. and has been found to hold up for as much as 40 hours. This compares with approximately two hours of protection afforded under similar test conditions by the conventional coating produced by a commercially utilized single bath process. These tests were run on both samples as taken from the bath and without any additional oil treatment. One reason for this high resistance to corrosion and rust is that the coatings produced by the multiple bath process of the present invention contain over approximately 40% of zinc consistently whereas the coatings produced by the conventional single bath process do not exceed approximately 30% of zinc and varied below this maximum over a wide range.

My invention has been illlustrated for zinc dihydrogen phosphate solutions and chloride ions. However, bromide ions and iodide ions can be used in place of the chloride ions.

The invention is also applicable to the coating of other metals besides iron and steel and alloy steels. It may, for example, be used for applying phosphate coatings to zinc, cadmium, and alloys thereof, particularly when an oxidizing agent such as a nitrate or chlorate is present.

While I have described certain preferred embodiments of my invention, many modifications thereof may be made without departing from the spirit of the invention; and I do not wish to be limited to the detailed examples, formulas and proportions of ingredients herein set forth, but desire to avail myself of all changes within the scope of the appended claims.

I claim:

1. The process of coating ferrous metal surfaces which comprises immersing a metal surface into a phosphate coating solution comprising essentially an aqueous acid phosphate coating solution containing zinc and ferrous iron and having a ratio of zinc to iron during operation of the process within the range of from approximately 1 to 1 :2, allowing the metal surface to remain in the solution until a substantial coating of phosphate is obtained, and removing the parts of a coated metal surface before the coating action is completed, before the solution on the surface is dried immersing said partially coated surface in another coating solution having a ratio of zinc to iron during normal operation within the range of from approximately 2:1 to 10:1 and removing the completely coated surface from the solution discarding a portion of the first solution prior to reaching an iron concentration exceeding 5 grams per liter in said first named coating solution, introducing a portion of the second solution to the first solution, and replenishing the second solution with a coating phosphate solution having substantially less iron content and a proportionately higher zinc content than the discarded portion of said first solution.

2. The process as claimed in claim 1 and further characterized in that the first solution contains zinc in an amount of approximately 0.8 gram per liter and ferrous iron in an amount of approximately 4 grams per liter.

3. The process of coating metal surfaces as claimed in claim 2 and further characterized in that the second solution contains ferrous iron in an amount of approximately 1 gram per liter and zinc in an amount of approximately 4.0 grams per liter.

4. The process of coating metal surfaces as claimed in claim 1 and further characterized in that the second solution contains ferrous iron in an amount of approximately 1 gram per liter and zinc in an amount of approximately 4.0 grams per liter.

5. A multiple step phosphate coating process for ferrous metals in which a plurality of separate baths are employed, each bath comprising an acid phosphate solution containing zinc and ferrous iron, the first bath having a higher iron content and a lower zinc content than do the subsequent baths, the novel steps of immersing a ferrous metal in said first bath until a substantial coating is formed on the surface of the metal, removing the coated metal surface from the bath before the coating action is completed, immersing said partially coated surface successively into subsequent baths before the first solution has dried on the surface and removing the completely coated surface from the bath having the lowest ferrous iron content and the highest zinc content.

6. A process as claimed in claim 5 and further characterized in that a portion of the coating solution is withdrawn from the first bath before sufiicient iron is dissolved from the metal to exceed the iron solubility of the solution and the withdrawn portion is replaced by solution from the next successive bath, which latter bath is then replenished with a fresh zinc acid phosphate coating solution.

7. A process as claimed in claim 5 and further characterized in that the ratio of zinc to iron in the first bath is maintained during normal operation in a ratio of from approximately 1:10 to 1:2 and the ratio of zinc to iron in the successive baths during normal operation is maintained within the range of from approximately 2 1 to 10:1.

8. A process as claimed in claim 5 and further characterized in that the first bath contains zinc in an amount of approximately 0.8 gram per liter and ferrous iron in an amount of approximately 4 grams per liter.

9. A process as claimed in claim 5 and further characterized in that the baths after the first bath contain ferrous iron in an amount of approximately 1 gram per liter and zinc in an amount of approximately 4.0 grams per liter.

10. A process as claimed in claim 5 and further characterized in that the first bath contains zinc in an amount of approximately 0.8 gram per liter and a ferrous iron in an amount of approximately 4 grams per liter and the successive baths contain ferrous iron in an amount of approximately 1 gram per liter and zinc in an amount of approximately 4.0 grams per liter.

11. A process as claimed in claim 5 and further characterized in that each bath initially contains the following materials in the following approximate per cents by weight; ZnO, .33%; HaPO4 1.40%; ZnClz, 0.20%; FeCh, .13%; water, 97.94% and which durin operation increases its iron content to approximately 4 grams per liter, the successive baths being maintained to keep the iron below approximately 1.5 grams per liter.

PAUL AMUNDSEN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,219,526 Allen Mar. 20, 1917 2,449,495 Lum Sept. 14, 1948 

5. A MULTIPLE STEP PHOSPHATE COATING PROCESS FOR FERROUS METALS IN WHICH A PLURALITY OF SEPARATE BATHS ARE EMPLOYED, EACH BATH COMPRISING AN ACID PHOSPHATE SOLUTION CONTAINING ZINC AND FERROUS IRON, THE FIRST BATH HAVING A HIGHER IRON CONTENT AND A LOWER ZINC CONTENT THAN DO THE SUBSEQUENT BATHS, THE NOVEL STEPS OF IMMERSING A FERROUS METAL IN SAID FIRST BATH UNTIL A SUBSTANTIAL COATING IS FORMED ON THE SURFACE OF THE METAL, REMOVING THE COATED METAL SURFACE FROM THE BATH BEFORE THE COATING ACTION IS COMPLETED, IMMERSING SAID PARTIALLY COATED SURFACE SUCCESSIVELY INTO SUBSEQUENT BATHS BEFORE THE FIRST SOLUTION HAS DRIED ON THE SURFACE AND REMOVING THE COMPLETELY COATED SURFACE FROM THE BATH HAVING THE LOWEST FERROUS IRON CONTENT AND THE HIGHEST ZINC CONTENT. 